Lord Saville's report on Bloody Sunday is bound to contain surprises when it is finally published. The exhaustive document will be handed to Shaun Woodward, the Secretary of State for Northern Ireland, in a week's time and the findings are likely to be made public a few days later.

If it concludes that the 14 people who were killed in Londonderry's Bogside on that fateful January day in 1972 had guns or bombs on them, it will create a sensation.

This would be because, firstly, all the years of hearings and hundreds of witnesses have failed to produce convincing evidence to back up allegations that those killed were gunmen and bombers. Secondly, two former British prime ministers, one Conservative and one Labour, have already exonerated those killed. In 1992, John Major said the dead "should be regarded as innocent of any allegation that they were shot whilst handling firearms or explosives". This was endorsed by Tony Blair when he set up the inquiry in 1998. In fact, the authorities conceded as far back as the mid-1970s that those shot by the Parachute Regiment were not gunmen or bombers.

Lord Saville will certainly take note of the fact that thousands of people were taking part in an illegal march, troops were attacked with stones and missiles, and several shots were fired by republicans. But no soldiers were killed or injured by gunfire or nail bombs, and no weapons were recovered by the Army. It seems out of the question Lord Saville will conclude there was anything that could come close to justifying the killings.

It is also unlikely that he will give any real credence to various myths, such as the assertion 34 gunmen and bombers were killed by the Army and spirited away for secret burials. That theory was effectively despatched at a hearing by one of the great emblematic figures of the Troubles, the now-retired Catholic bishop Fr Edward Daly. On Bloody Sunday he was a young local priest, crouching and desperately waving a blood-stained handkerchief, seeking safe passage from troops as a fatally injured youth was carried away. That image will endure in Irish history.

At the hearing, counsel for more than 400 soldiers commended the churchman's honesty and what was described as his record of outspoken opposition to violence. Asked about the supposed secret burials, he gave a curt response: "I think it is offensive nonsense." He also gave an insight into the lasting effects of Bloody Sunday when he described visiting young people in prison. Many of them told him, he recalled, that they would never have become involved in the IRA if it had not been for the events of that day. Gerry Adams of Sinn Fein confirmed in his memoirs how it had swelled republican ranks. "Money, guns and recruits flooded into the IRA," he wrote.

In 1972, the city had a particularly dangerous Provisional IRA unit whose prime mover, Martin McGuinness, was as effective as an IRA commander as he is today as a Government minister. His units pulverised so many premises that the city "looked as though it had been bombed from the air". During the two-years when he was in command or second-in-command, the organisation killed 29 members of the security forces, 24 of them regular soldiers.

After some hesitation, Mr McGuinness agreed to give evidence to the tribunal. On Bloody Sunday, he was the organisation's second-in-command; afterwards he took over as its "Officer Commanding". He told the tribunal: "There was a state of war between the IRA and British military forces. This was a war area."

The probability is that the Saville tribunal will not reject his testimony. On the morning of Bloody Sunday, according to his account, he arose at about 9am, went to Mass and locked away his guns and bombs. "Certainly the IRA had nail bombs, but not in that area," he said. "It would have been lunacy of the worst kind for anyone to have nail bombs when 30,000 people were on the street."

He went unarmed to the march on what turned out to be "the worst day that I had ever experienced in my life". It was, he said, "devastating – I was in a daze".

The IRA did not engage the Army, he insisted. He said: "I felt helpless, angry and disgusted that there was nothing I could do. I wanted to get a rifle, find other Volunteers and try to do something."

At that time, political direction was divided between the Cabinet of the Prime Minister, Edward Heath, and a unionist administration at Stormont. Documents unearthed by the tribunal showed the two governments feared a general breakdown of law and order. But Bloody Sunday made a bad situation worse, as part of a further escalation of violence which meant that, with almost 500 dead, 1972 was to be by far the worst year of the Troubles.

One theory is that squaddies ran amok, out of the control of ineffective Parachute Regiment officers. Another is that the military top brass wanted "to teach the IRA a lesson". Yet another is that Mr Heath's Cabinet specifically instructed the Army to take the gloves off. But it is difficult to believe the conclusion will be that anyone in high authority set out to leave civilian bodies on the streets: it was obvious that such killings were highly damaging to the cause of anyone who caused them.

This was illustrated by the huge reaction to the shootings, locally and internationally, and helps explains why the Heath Government was galvanised into setting up an immediate inquiry under Lord Widgery, then the Lord Chief Justice. The judge concluded only that the actions of some soldiers "bordered on the reckless", prompting widespread accusations that this amounted to a cover-up. As Tony Blair's chief of staff, Jonathan Powell was involved in the decision to set up the Saville inquiry. He said that when he read the Widgery report "it was perfectly clear that it was a complete and utter whitewash". Lord Widgery was as brisk as Lord Saville has been unhurried and meticulous. He delivered his report in what some thought almost indecent haste, finishing the task in just 11 weeks. The document's text ran to just 39 pages, with some of the deaths rating just one paragraph each. The judge also studiously refrained from reading hundreds of eyewitness reports.

The report was viewed as adding insult to injury. As the Irish Government put it: "The victims suffered a second injustice, this time at the hands of Lord Widgery, who sought to taint them with responsibility for their own deaths in order to exonerate those he found it inexpedient to blame." The Saville exercise was inordinately time-consuming and expensive. Even as it proceeded, new legislation was introduced to ensure that no future inquiry would ever again devour such resources. Jonathan Powell was astounded when even Martin McGuinness remarked to him that "an apology would have been quite sufficient".

Northern Ireland has, of course, been transformed since 1972. Today Mr McGuinness is second-in-command of Northern Ireland's Government. But Bloody Sunday is still part-history, part-current affairs. Lord Saville has a much better chance of producing a report that might command the sort of widespread acceptance that Lord Widgery's never did. Yet it is unlikely that his conclusions will satisfy everyone: those who are unshakeably convinced that British ministers ordered a deliberate massacre of innocent civilians will not be satisfied. There will certainly be much debate on a venture which has generated such a gigantic amount of documentation. In paper form the material is said to fill a lorry.

No one yet knows, however, whether this colossal exercise will unearth what most can accept as the truth, bring closure to the bereaved, and at last lay to rest the ghosts of Bloody Sunday.

In numbers: The Saville Inquiry

* The tribunal has been the longest and costliest inquiry in British legal history, with a totalcost of almost £200m.

* The inquiry sat for 434 days between 2000 and 2004.

* 2,500 witness statements were collected, with more than 900 people called to give direct evidence. Those interviewed included more than 600 soldiers and 700 civilians.

* An estimated 16 million words were spoken during the oral hearings.